Debug the kernel:
1.Compile the kernel source
  Go to kernel_source folder;
  Run "make";
  One should see "mk obj/kern/kernel.img". This means the compilation succeeded. 
2.Run kernel image
  Stay in kernel_source folder;
  Run "make qemu-nox-gdb";
3.Debug kernel
  Open a new window and stay in kernel_source folder;
  Run "gdb"
  One should see "Remote debugging using localhost:26000
                  0x0000fff0 in ?? ()"
4.The file .gdbinit is the config for computer archetectur and auto-reload. It
  should be placed under /home/lizhi/

Tips for QEMU:
1.Quit: ctrl+a x